Image or optical scanners are enjoying ever-growing utility in a variety of products and for a diversity of applications. For the purpose of understanding the subject invention, "image or optical scanners" are defined as including one or more photoresponsive circuits or elements operatively disposed so as to optically scan a pattern of data and generate a detectable signal representative of the scanned pattern.
Optical scanners may be readily adapted to address a wide variety of data inputs. The data may be in the form of a photograph, a drawing, a design on fabric or the like or any other such graphic patterns. In other forms, the data being scanned may be alpha-numeric data such as printed or written matter. Regardless of the form of data, the image scanners are adapted to convert a pattern of data into an electrical signal which may be supplied to downstream apparatus for further processing, storage or display. For example, image scanners have been incorporated into facsimile machines, copying machines, computer input terminals, CAD-CAM systems and the like. Additionally, image scanners are utilized in production processes to inspect the surfaces of materials such as plywood, fabric, and metal. The typical image scanner includes one or more photosensitive pixels disposed so as to either simultaneously, or sequentially address various portions of the surface being scanned.
There are several approaches currently employed for the fabrication of image scanners. Charge coupled devices (CCD's) form the basis for one such approach. CCD's are solid state devices, typically formed from crystalline silicon and including therein a plurality of photoresponsive circuits each having a pixel associated therewith. CCD's have a high degree of photosensitivity and are capable of providing high resolution. However, CCD's are relatively small in size; the typical CCD array is approximately one inch in length, and the largest CCD's currently produced are approximatey 3 to 4 inches in length. These size constraints impose restrictions on the utility of CCD's in scanners. In those instances where a pattern of information having dimensions larger than that of the CCD is being scanned, an optical system must be utilized to project that pattern of information at a reduced size onto the surface of the CCD. Aside from being expensive and bulky, such optical systems will effectively reduce the resolution of the CCD.
Thin film devices represent another approach to the fabrication of image scanners. Thin film devices may be formed by vapor deposition of layers of appropriate semiconductor materials onto a variety of substrates. By appropriately patterning these layers, a variety of device configurations may be fabricated.
Recently, considerable progress has been made in developing processes for depositing thin film semiconductor materials. Such materials can be deposited to cover relatively large areas and can be doped to form p-type and n-type semiconductor materials for the production of semiconductor devices such as p-i-n type photodiodes equivalent, and in some cases superior to those produced by their crystalline counterparts. One particularly promising group of thin film materials are the amorphous materials. As used herein, the term "amorphous" includes all materials or alloys which have long range disorder although they may have short or intermediate range order, or even contain at times crystalline inclusions. Also as used herein, the term "microcrystalline" is defined as a unique class of said amorphous materials characterized by a volume fraction of crystalline inclusions, said volume fraction of inclusions being greater than a threshold value at which the onset of substantial changes in certain key parameters such as electrical conductivity, band gap and absorption constant occur.
It is now possible to prepare by glow discharge, or other vapor deposition processes, thin film amorphous silicon, germanium or silicon-germanium alloys in large areas, said alloys possessing low concentrations of localized states in the energy gap thereof and high quality electronic properties. Techniques for the preparation of such alloys are fully described in U.S. Pat. Nos. 4,226,898 and 4,217,374 of Stanford R. Ovshinsky, et al., both of which are entitled "Amorphous Semiconductor Equivalent to Crystalline Semiconductors" and in U.S. Pat. Nos. 4,504,518 and 4,517,223 of Stanford R. Ovshinsky, et al., both of which are entitled "Method of Making Amorphous Semiconductor Alloys and Devices Using Microwave Energy"; the disclosures of all of the foregoing patents are incorporated herein by reference.
Thin film alloys may be readily manufactured in large areas by mass production processes and therefore enable the economic manufacture of large scale image sensor arrays. Use of such large arrays eliminates the need for complicated optical systems thereby effecting savings in cost, product size and processing steps. Additionally, since the thin film sensor arrays are fabricated to be of approximately the same size as the object being scanned, relatively high resolution may be attained without the necessity of employing high resolution photolithographic processing steps. It may thus be seen that thin film photosensor arrays have significant utility in the fabrication of image scanners.
Typical thin film image scanners include an array of photoresponsive circuits, each of which incorporate therein a photogenerative element adapted to provide an electrical signal corresponding to the quantity of light incident thereupon. It would be very time consuming to utilize a single element for scanning, accordingly, an array of elements in either linear or two dimensional form is typically utilized. In those instances where such an array is employed, each photosensitive circuit of the array must also include a blocking element such as a diode or transistor. The blocking element facilitates addressing of the various photogenerative elements in the matrix by preventing current flows through unwanted paths in the matrix. In this manner, the blocking device eliminates cross talk which would otherwise degrade the signal produced by the photosensitive element.
Problems can occur in the use of photosensitive arrays because of the generation of charge carrier pairs within the blocking element thereof due to the absorption of incident illumination. Light having an energy greater than the band gap of the semiconductor material from which the blocking element is fabricated is capable of generating an electron-hole pair in that material. If a field is present across the semiconductor material, the electron-hole pair is separated, thereby generating a flow of electrical current. Such illumination can produce a flow of electrical current which will effectively be a source of "noise" which dissipates or otherwise degrades the signal produced by and hence the sensitivity of the photosensitive elements. This problem has heretofore been dealt with in several manners. According to one approach, the blocking element has been masked with an opaque material so as to prevent ambient light from striking it. While this approach does solve the problem, it necessitates additional processing steps and wastes valuable real estate which could otherwise be utilized to improve resolution in a two dimensional matrix.
Because of the shortcomings of the masking approach, a new pixel configuration and driving scheme has been developed which eliminates the need for covered blocking elements, the details of which are disclosed in U.S. Pat. Application Ser. No. 907,926 filed Sept. 16, 1986, now U.S. Pat. No. 4,714,836 issued Dec. 22, 1987 and entitled "Photosensitive Pixel With Exposed Blocking Element", the disclosure of which is incorporated herein by reference. As detailed in the foregoing application, the blocking element of a pixel need not be shielded from incident radiation, since photocurrents generated therein will not discharge or otherwise dissipate the signal produced by the photogenerative element. It has, in fact, been found by the instant inventor that when using the pixel and driving scheme of the aforementioned application, photocurrents produced by the blocking element will actually contribute to those produced by the photogenerative element.
However, the photocurrent provided by the blocking element can, in some instances, be a source of unwanted signal degradation, even though it does not actually discharge the signal from the photogenerative element. This is because of the phenomenon known as "optical cross-talk." Optical cross-talk occurs when the blocking element and the photogenerative element of a pixel are illuminated by radiation emanating from different portions of the image being sensed. If the photogenerative element is illuminated by radiation from a dark image portion, while the blocking element is illuminated by radiation from a light image portion, the signal generated by the blocking element will attenuate the signal generated by the photogenerative element thereby reducing signal sensitivity.
Optical cross-talk can thus be seen to be a problem which can degrade the sensitivity of a photosensitive pixel. Obviously, the problem can be eliminated by masking the blocking element; however, such a solution is not preferred for the reasons discussed previously. Therefore, it will be appreciated that there is a need for a photosensitive pixel having an unmasked blocking element, which pixel is not susceptible to problems of optical cross-talk.
Furthermore, it is also desirable to maximize the geometrical packing density of pixels in an array so as to maximize the resolution of that array. The present invention provides for an improved photosensitive pixel including a blocking element and a photogenerative element in which the respective sizes and shapes of the elements cooperate so as to substantially eliminate optical cross-talk and to maximize the geometric packing density of the pixels when disposed in a photosensitive array.
These and other advantages of the instant invention will be apparent from the brief summary of the invention. The drawings and detailed description thereof and the claims which follow.